Did you only weld one pipe or both? Pics would probably help.

I've noticed that Motordyne, ARK, and HKS all use Helmholtz resonators that look much smaller than 29" to cancel their drone. Seems like it works. Does anyone know why? And if resonators like that would work in drone reduction when welded onto any other exhaust? It looks much nicer than having long tubes imo.

Found this thread while searching for cures for exhaust drone. Gave it a try today and the results are better than I expected. I've got a 2010 NISMO with HFCs and Beluga axle backs. The goal was to reduce the drone between 2500 to 3000rpm. I had the muffler shop install a side branch resonator on each side just before the axle back flanges. Using the formula found in this thread, each branch length is 28in, using 2.25in pipe and capped at the ends. I'm very happy with the results, as the drone at the target rpm range has been reduced by about 80%, and the entire rpm range sounds smoother.

I'm sure others have done this as well as this thread is pretty old. I wasn't aware of this method and just wanted to report that it does work. I know that some aftermarket exhausts come with the resonators, but never thought of fabricating them on exhausts that didn't include them.

The job cost me $125 including parts and labor.

Thanks for the write-up. Just installed my FI exhaust today and the drone is a deal killer. Looking at options.

I would think this would also work if it runs parallel to the mid pipes.

Good stuff! Wish I saw this before I sold my Top Speed axle back. I had the exact same setup as you and LOVED the sound but cabin noise was too much.

Sold the axle back and got CNT cat back which had the helmholz canisters to eat drone, much like your added pipes. It is now good but your current setup is probably better!

Actually, even though the Helmholtz resonators work on reducing drone, the drone isn't really caused by the exhaust.

Drone is caused by the frequency produced at those lower rpms and the natural resonant frequency of the interior air space of the car are tuned to one another, reinforcing those frequencies, making them louder. And if you notice, the drone comes back at double those rpms, around 4000 to 5000. The drone at low rpms is like the first harmonics, the drone at higher rpms are like the second harmonics. This is because the internal air volume of the car interior allows those particular frequencies to propagate - a.k.a. - drone.

Ever notice the drone decreases drastically to almost non-existent when you have the windows rolled down? That's because you just changed the "tuned" resonant frequency of the interior by opening the windows.

Has a trusted friend or family member drive off with your car as you stand outside and listen. I bet you won't hear any drone. I bet the exhaust will sound just about a loud at those low rpms (2000 - 2800) as it does at higher rpms (3000 - 5000).

A very similar thing happens in a room with bass notes on a stereo system. It's called "room gain" and "room modes" which have both peaks and nulls. Different room sizes will have different room modes where different bass frequencies are allowed to propagate near the wall boundaries making them louder or possibly quieter. In certain areas along the walls and within the interior of the room, you'll notice the bass frequencies are louder than normal, typically near the corners (a.k.a. - corner loading).


Anyway, now I'm starting to ramble, so I'll stop there. But my point is this, it's NOT the exhaust system that's droning, it's the enclosed interior of the car that's causing the drone.

I agree with you right up to the point it's not the exhaust causing it. If so, then why does the drone stop when the engine is off or you change exhaust?

Please re-read my post. It's explained in there.

Based on your understanding, I do not think you mean to suggest that different exhausts do not produce different frequencies. The exhaust is most certainly a key variable of the drone. Not that there aren't other factors like you explain. But to suggest that the exhaust is irrelevant to drone is wrong.

I just started on this today. Invidia gemini with FI resonated hfc's. Its not bad but its riiiight at 2800 to 2900 rpm...75to 80 mph, perfect cruising speed. So ive welded caps on so far. I wanted to package this so they sat above the mufflers, not adding extra weight to the middle, so the pipes arent an ideal length. One is 21and the other is 26. Maybe itll cut out more frequiencies because of the different lengths, I dont know. Im using 2.5 in. tubing, so the volume of the space created is pretty big. [IMG]http://i1059.photobucket.com/albums/...pseqndp4ye.jpg[/IMG]

so I had my Injen exhaust installed yesterday (very similar to the FI in layout) with HFC's and there is noticeable drone between around 1800-2900 RPMs. I downloaded a realtime sound analyzer app, and my resonant peaks at around 131-136 Hz, which is about a 25" pipe. I took it to a "performance exhaust" muffler shop that advertises that they do "custom" exhausts and even showed him a picture (from this thread) and a drawing that I made. He told me he couldn't do it, one because there wasn't enough space (Looked like plenty to me) and two, because the exhaust is SS. He said he couldn't weld SS...????

Anyway, the drone has already gotten a little better just from driving and putting a little mileage on the HFC's and mufflers, so I'm hoping it will get a good bit better on its own, but if I do need to take this route in the future, does anyone else know of muffler shops that can't weld SS??? Is this an anomaly, or is that pretty common?

It's hit or miss, but have heard of shops that can't weld SS. It has to do with the equipment and materials they have.

My local muffler shop was able to weld onto the SS FI exhaust system I have.

Added 28" branch pipes 2 1/4" diameter pipes just before the mufflers and it has quieted down the drone a good amount. I still get drone right at 2K rpm but higher up it a lot better.

Also added a foam ensolite in the trunk to help.

Problem is now I hear some rattling from something that's loose near the muffler.

do you have any idea what your peak resonant frequency is?

No idea, 28 in was the max that I could fit in the back right before the muffler cans. From reading this thread from other car forums, 28" seemed like a good start for the rpm range that I was trying to fix. The vibration or "drone" at 2500 - 2900 rpm on the freeway was giving me headaches. I was ready to have them cut and reweld to different lengths if needed but it worked out for my purposes.

Best advice is talk to your muffler shop to be ready to adjust as needed. Someone mentioned using a sleeve and clamp setup to find the best length for your purposes. It will cost a little more to test and tune it but if it's really killing your driving experience then I think it's worth it.

Theoretical calculations are one thing but driving experience is best.

right, but I was wondering why you still had drone at 2k... the longer the resonators, the lower the frequency they kill, right? so I was wondering how the calcs worked out (I'm a mechanical engineer in real life - doesn't mean much, but I like to think about this stuff)

Sorry, I used the trial and error method. :)

EDIT: Maybe because my side pipes are just 2.25" in diameter and should be 2.5" to match the exhaust diameter in theory.